[firebase_messaging]: Remove references to deprecated v1 Android embedding about flutterfire

[reidbaker]

https://github.com/search?q=repo%3Afirebase%2Fflutterfire%20(%22io.flutter.view.FlutterMain%22%20OR%20%22io.flutter.view.FlutterNativeView%22%20OR%20%22io.flutter.view.FlutterView%22%20OR%20%22io.flutter.embedding.engine.plugins.shim%22%20OR%20%22io.flutter.app%22%20OR%20%22PluginRegistry.Registrar%22)&type=code

The only instances in this plugin are dart docs.

[reidbaker]

Confirmed compilation against the version of the engine with the classes removed.

[Lyokone]

Thanks for checking @reidbaker, if I understand properly, the only thing that we still have to do is remove the references in documentation?

[reidbaker]

Correct. I debated not filing an issue at all since it wont cause issues for your customers but figured you might want to know you are referencing classes that will not exist in the future :).

[Lyokone]

Thanks for taking the time to check :)